The molecular events involved in
tumor cell death induced by novel photoproducts of
merocyanine 540 (pMC540) are poorly understood. Using HL60
leukemia and M14
melanoma cell lines we investigated the role of the apoptotic pathway in pMC540-mediated cell death.
Tumor cells exposed to pMC540 showed cell size shrinkage and an increase in the sub-diploid
DNA content. A loss of membrane
phospholipid asymmetry associated with apoptosis was induced by pMC540 in both tumor cell lines as evidenced by the externalization of
phosphatidylserine. A dose-dependent increase in
caspase-3 protease activity suppressed by the tetrapeptide inhibitor
DEVD-CHO was observed in both cell lines. Western blot analysis of
poly (ADP-ribose) polymerase, a
caspase substrate, showed the classical cleavage pattern (116 to 89 kDa) associated with apoptosis in pMC540-treated cell lysates. Furthermore,
caspase inhibition blocked the externalization of membrane PS, indicating that the loss of membrane
phospholipid asymmetry is a downstream event of
caspase activation. These findings demonstrate that
tumor cell death induced by pMC540 is mediated by
caspase proteases.